Add a `platform=list` SCons argument to list platforms
Add a `platform=list` SCons argument to list platforms

This also improves the message display and makes an invalid selection
exit with a non-zero status code.

vnen/godot,vnen/godot,DmitriySalnikov/godot,sanikoyes/godot,godotengine/godot,ex/godot,BastiaanOlij/godot,akien-mga/godot,Zylann/godot,BastiaanOlij/godot,akien-mga/godot,vkbsb/godot,Shockblast/godot,Valentactive/godot,pkowal1982/godot,pkowal1982/godot,josempans/godot,MarianoGnu/godot,ex/godot,Valentactive/godot,Zylann/godot,Zylann/godot,honix/godot,Paulloz/godot,BastiaanOlij/godot,Zylann/godot,vkbsb/godot,Faless/godot,firefly2442/godot,ex/godot,ZuBsPaCe/godot,godotengine/godot,Valentactive/godot,sanikoyes/godot,DmitriySalnikov/godot,pkowal1982/godot,DmitriySalnikov/godot,vnen/godot,Shockblast/godot,BastiaanOlij/godot,firefly2442/godot,Shockblast/godot,josempans/godot,godotengine/godot,sanikoyes/godot,pkowal1982/godot,guilhermefelipecgs/godot,josempans/godot,firefly2442/godot,DmitriySalnikov/godot,firefly2442/godot,akien-mga/godot,ex/godot,ZuBsPaCe/godot,godotengine/godot,vnen/godot,DmitriySalnikov/godot,Valentactive/godot,godotengine/godot,Paulloz/godot,guilhermefelipecgs/godot,Faless/godot,firefly2442/godot,vnen/godot,firefly2442/godot,guilhermefelipecgs/godot,Zylann/godot,akien-mga/godot,MarianoGnu/godot,ex/godot,honix/godot,Shockblast/godot,vkbsb/godot,Shockblast/godot,akien-mga/godot,honix/godot,vkbsb/godot,akien-mga/godot,vkbsb/godot,vnen/godot,honix/godot,Valentactive/godot,vnen/godot,guilhermefelipecgs/godot,Shockblast/godot,pkowal1982/godot,josempans/godot,BastiaanOlij/godot,Paulloz/godot,vkbsb/godot,josempans/godot,godotengine/godot,Paulloz/godot,ZuBsPaCe/godot,ex/godot,Faless/godot,Faless/godot,MarianoGnu/godot,Zylann/godot,Paulloz/godot,Paulloz/godot,Zylann/godot,akien-mga/godot,Paulloz/godot,Zylann/godot,sanikoyes/godot,ex/godot,Valentactive/godot,MarianoGnu/godot,DmitriySalnikov/godot,josempans/godot,guilhermefelipecgs/godot,MarianoGnu/godot,akien-mga/godot,Faless/godot,Faless/godot,MarianoGnu/godot,pkowal1982/godot,sanikoyes/godot,ZuBsPaCe/godot,pkowal1982/godot,vkbsb/godot,Faless/godot,honix/godot,ZuBsPaCe/godot,MarianoGnu/godot,josempans/godot,Shockblast/godot,ZuBsPaCe/godot,firefly2442/godot,pkowal1982/godot,sanikoyes/godot,sanikoyes/godot,guilhermefelipecgs/godot,sanikoyes/godot,Faless/godot,BastiaanOlij/godot,BastiaanOlij/godot,ZuBsPaCe/godot,Valentactive/godot,josempans/godot,ZuBsPaCe/godot,ex/godot,BastiaanOlij/godot,godotengine/godot,godotengine/godot,Valentactive/godot,firefly2442/godot,guilhermefelipecgs/godot,vkbsb/godot,DmitriySalnikov/godot,honix/godot,guilhermefelipecgs/godot,MarianoGnu/godot,Shockblast/godot,vnen/godot